Iterative-structure digital signal reception device, and module and method therefor

ABSTRACT

The invention relates to a device for the reception of signals formed by a series of digital symbols corresponding to the convolutive encoding of items of source digital data comprising p cascade-connected detection and decoding modules M 1  to M p , p being greater than or equal to 2, each of said modules M i  including: (a) inter-symbol interference correction means supplied by a symbol input R i  and delivering estimated symbols A i ,1 with weighted values, (b) means for the decoding of said estimated symbols A i ,1, performing operations symmetrical to said convolutive encoding and delivering decoded symbols A i ,2 with weighted values, and (c) means for the computation of an item of correction information Z i+1  from said estimated symbol A i ,1 and said decoded symbol A i ,2, (d) said means for the correction of each of said modules M i , except for the first module M 1 , taking account of at least one item of correction information Z i  determined by the previous module M i-1 .

BACKGROUND OF INVENTION

The field of the invention is that of the transmission and broadcastingof digital signals, especially in the presence of transmission noises.More specifically, the invention relates to the reception of digitalsignals encoded by means of a convolutive type error correction code.

The invention can be applied in all cases where digital signals aretransmitted or broadcast on noise-infested channels. For example, thepresent invention can be implemented in receivers of systems for digitalradiocommunications with mobile units such as the GSM system.

Other possible applications are, for example, the reception of signalstransmitted by RF channels, satellites, etc. More generally, theinvention can be applied advantageously in all cases where a convolutivetype code is implemented at transmission.

In the known type of receivers, the symbols retrieved at output of thedemodulator take the form of analog samples which, after quantification,are generally processed by an equalizer (a transversal filter) given thetask of eliminating the inter-symbol interference introduced by thechannel. The equalized samples are then de-interleaved if necessary anddecoded before being given to the addressee.

The principle of equalization consists in estimating the response of thetransmission channel for the application, to the received signal, of afiltering operation symmetrical to this response so as to: obtain acorrected signal. In particular, the equalization is aimed ateliminating or, at least, at limiting the inter-symbol interferenceintroduced by the channel.

The response of the channel is generally estimated by the analysis of areference signal known to the receivers. Of course, the transmission ofthese reference signals leads to a reduction of the useful bit rate.

The equalized items of data are then de-interleaved (in an operationsymmetrical to the interleaving done at the time of encoding, if such aninterleaving is planned) and then decoded.

In certain situations, it happens that the equalization of this type isnot sufficient to ensure high quality source decoding.

The article by P. Jung and P. W. Baier, "VLSI implementation of softoutput Viterbi equalizers for mobile radio applications" (Proc. of IEEE42nd Vehicular Technology Society Conference, Denver, Colo., pp.577-585, May 1992) proposes the use, instead of a transversal filter, ofa symbol detector working according to the principle of maximumlikelihood.

This technique is more efficient if the coefficients representing thechannel are properly estimated. However, once again, the equalized itemsof data may prove to be of insufficient quality. Furthermore, it is morecomplicated to make and requires more space than with the transversefilter technique.

Convolutive codes are codes that associate at least one encoded item ofdata with each item of source data to be encoded. This encoded item ofdata is obtained by the modulo 2 summation of this item of source datawith at least one of the previous items of source data. Thus, eachencoded signal is a linear combination of the item of source data to beencoded and previous items of source data taken into account.

In the decoder, the original items of data are most usuallyreconstructed by means of a maximum likelihood algorithm, for examplethe Viterbi algorithm, whose decisions may be weighted if necessary. TheViterbi algorithm, in taking account of a sequence of received encodedsymbols, provides an estimation of each item of data encoded attransmission, in defining the source sequence most probablycorresponding to the received sequence.

The Viterbi algorithm may also be used to detect sequences affected byinter-symbol interference. The invention can be also applied in thiscase.

Clearly, the greater the number of symbols taken into account, the morereliable is the decision. By contrast, the greater this number, the morecomplicated is the decoder or the detector (hereinafter, the termdecoder is used to describe decoders themselves as well as detectors.This observation is applicable also to the term decoding which must beunderstood to mean decoding or detection as the case may be. The memoryspace needed soon becomes very great, as do the correspondingcomputation times.

The integrated circuits that implement such algorithms therefore mostusually rely on a compromise between cost and performance. Theseindustrial-level choices do not always enable the construction ofdecoders that correspond in an optimum way to a given application. Forexample, it is not possible to make low-cost decoders for applicationswhere the reception quality is not of crucial importance, as integratedcircuits cost too much. Conversely, these integrated circuits too arenot suited to the making of receivers with very high decoding qualityfor which the cost price is of little importance.

An advantageous example of convolutive encoding, to which the inventioncan be applied, is described in the French patent application FR 9105280 filed on behalf of the same Applicants as well as in the articleby C. Berrou, A. Glavieux and P. Thitimajshima, "Near Shannon limiterror-correcting coding and decoding: turbo-codes" (Proc. ICC'93, pp.1064-1070, Geneva, Switzerland, May 1993). This class of codes is knownin particular as "turbo-codes".

The elementary codes (recursive systematic or pseudo-systematic codes)described in the French patent application FR 91 05278 filed on behalfof the present Applicants may also be used.

The invention is aimed especially at overcoming the drawbacks of theprior art reception device and more specifically at improving thecorresponding performance characteristics.

More specifically, a goal of the invention is to provide a device ofthis kind with very high corrective capacity as compared with knownmethods presently used in digital communications systems.

The invention is aimed in particular at providing methods of this kindthat are particularly efficient, again with respect to known methods,for transmission in highly noise-infested channels.

It is also an aim of the invention to provide a device of this kind thatis highly efficient but nevertheless easy to manufacture on anindustrial scale at acceptable costs.

Thus, a particular aim of the invention is to provide a decoding methodenabling implantation on a silicon surface that is small enough for itsindustrial-scale manufacture to be possible, for example, on a surfacearea smaller than 50 mm².

It is also an aim of the invention to provide a method of reception usedfor the making of numerous types of receivers with performancecharacteristics and cost price that vary as a function of the needsfulfilled, implementing one or more integrated circuits of a singletype.

In other words, an essential aim of the invention is to provide methodsof this kind enabling firstly profitable industrial-scale manufacturebased on the development of a single and relatively simple integratedcircuit and, secondly, the making of receivers that can be used for avery wide variety of applications.

BRIEF SUMMARY OF THE INVENTION

These goals as well as others that shall appear hereinafter are achievedaccording to the invention by means of a device for the reception ofsignals formed by a series of digital symbols corresponding to theconvolutive encoding of items of source digital data comprising pcascade-connected detection and decoding modules M₁ to M_(p), p beinggreater than or equal to 2, each of said modules M_(i) including:

inter-symbol interference correction means supplied by a symbol inputR_(i) and delivering estimated symbols A_(i),1 with weighted values,

means for the decoding of said estimated symbols A_(i),1, performingoperations symmetrical to said convolutive encoding and deliveringdecoded symbols A_(i),2 with weighted values, and

means for the computation of an item of correction information Z_(i+1)from said estimated symbol A_(i),1 and said decoded symbol A_(i),2,

said means for the correction of each of said modules M_(i), except forthe first module M₁, taking account of at least one item of correctioninformation Z_(i) determined by the previous module M_(i-1).

Thus, according to the invention, the standard function of equalization(filtering) is replaced by a symbol detector that takes account of thememory effect introduced by the channel.

In other words, the invention proposes a completely novel and optimizedapproach to the correction of inter-symbol interference. Indeed,conventionally, the equalization does not take account of the items ofdata transmitted but only of the channel estimation. By contrast, theinvention is based on a dynamic approach, in taking account of the lastsymbols received to improve the equalization.

In other words again, the invention, in its general principle, likensthe effect of inter-symbol interference to a particular convolutiveencoding (except that there is no redundancy in the case ofinterference: to a symbol emitted on the channel, there corresponds oneand only received symbol), and carries out its decoding. Morespecifically, the phenomenon of inter-symbol interference introduced bythe channel may be represented by a lattice. This lattice differs from aconvolutive encoding lattice by the presence of multiplier coefficientsthat vary in time. Thus, each module of the invention includescorrection means and decoding means having similar structures based onconvolutive decoding techniques.

The technique of the invention therefore consists in computing an itemof correction information that represents the received symbol and intaking account of one or more of these items of information to correctthe interference.

It will be understood that the taking into account of this informationelement dictates an iterative reception structure because of the latencyintroduced by the processing operations. Thus, during the firstiteration, a first item of correction information is computed. Thisinformation element is used and, as the case may be, it is recomputedduring the following iterations. According to the invention, theiterative approach takes the form of a structuring of the receiver(detector of symbols and decoder) in the form of cascade-connectedmodules with each module corresponding to an iteration. This is acompletely novel approach to the correction of inter-symbolinterference.

Advantageously, these modules are all identical. In particular, they maytake the form of integrated circuits.

The greater the number of modules, the greater is the increase in theefficiency of reception (correction of inter-symbol interference anddecoding). The modular approach of the invention can be used to easilyobtain several types of receivers corresponding to different levels ofreception quality, as a function of the modules implemented. Similarly,it is possible to provide for decoders with an open-ended structure towhich modules may be added as needed.

DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention shall appear from thefollowing description of a preferred embodiment of the invention givingby way of a non-restricted example and from the appended drawings, ofwhich:

FIG. 1 is a block diagram illustrating a standard transmission line thatcan incorporate the reception device according to the present invention.

FIG. 2 shows a simplified view of a discrete model, equivalent in termsof baseband to a transmission channel affected by an inter-symbolinterference, illustrating the approach of the present invention.

FIG. 3 shows the corresponding lattice of the channel of FIG. 2,illustrating possible developments of the state of the channel as afunction of time.

FIG. 4 is a block diagram showing the principle of the receiver of thepresent invention.

FIG. 5 is a simplified flow chart illustrating the method of the presentinvention implemented in the receiver of FIG. 4.

FIG. 6 illustrates the modular structure of the receiver of the presentinvention in the case of four cascade-connected modules.

FIG. 7 shows a block diagram of the module of FIG. 6.

FIG. 8 shows a series of curves illustrating the performancecharacteristics of the present invention in the case of a Gaussianchannel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a preferred embodiment of the invention, each of saidmodules M_(i) has two inputs, one symbol input R_(i) and one correctioninput Z_(i) and three outputs, one symbol output R_(i+1), one correctionoutput Z_(i+1) and one decoded symbol output D_(i+1),

the symbol input R_(i) of the module M_(i) being connected to the symboloutput R_(i) of the module M_(i-1) for i greater than 1 and beingsupplied with the symbols received for i equal to 1,

the correction input Z_(i) of the module M_(i) being connected to thecorrection output Z_(i) of the module M_(i-1) for i greater than 1 andbeing supplied with a neutral value that does not affect the correctionfor i equal to 1,

the symbol output R_(i+1) of the module M_(i) being equal to the symbolinput R_(i) of said module M_(i) delayed by the latency time dictated bythe module M_(i),

the correction output Z_(i+1) delivering said correction information,and the decoded symbol output D_(i+1) being unused for the modules M₁ toM_(p-1) and being supplied with the decoded symbol A_(p),2 of the moduleM_(p) if the code implemented is a systematic code and, if not, with acorresponding reconstituted value.

This structure, as indicated further above, makes it possible to designidentical modules. Of course, depending on whether it is the firstmodule, the last module or an intermediate module, the inputs andoutputs used may be different.

Advantageously, said means for the correction of the inter-symbolinterference are supplied with an item of correction information Z_(i)representing a difference between each decoded symbol A_(i),2 and thecorresponding estimated symbol A_(i),1.

In this case, preferably, said means to compute the item of correctioninformation comprise means for the multiplication of said estimationsymbols A_(i),1 by a positive weighting coefficient γ₂ deliveringweighted symbols, and subtraction means computing the difference betweeneach decoded symbol A_(i),2 and the corresponding weighted symbol anddelivering said item of correction information Z_(i).

In particular, the value of this coefficient is set in such a way thatthe information borne by Z_(i) no longer contains any informationrelating to A_(i),1 so that the noise affecting Z_(i) and the receivedsamples R_(i) are decorrelated. Thus the information element reinjectedinto the input of the equalizer is "new" as compared with the receivedsamples (it comes from the decoding process). It shall be called"extrinsic information".

According to an advantageous embodiment, said means for the correctionof inter-symbol interference perform the following operations:

the computation of a correction value V_(i) =γ₁ |Z_(i) |, where γ₁ is apositive coefficient and where | | represents the absolute valueoperator;

for each arm of the lattice corresponding to said convolutive code, thedetermining of a detected symbol A_(i),o, comprising the followingsteps:

if the symbol assigned to said branch has the same sign as said item ofcorrection information Z_(i), the subtraction, from the metricassociated with said arm, of said correction value V_(i) ;

if the symbol assigned to said arm has a sign opposite that of said itemof correction information Z_(i), the addition, to the metric associatedwith said arm, of said correction value V_(i) ;

the subtraction, from said detected symbol A_(i),o, of the value γ_(i)Z_(i).

An object of the latter subtraction is to "rid" the output of the symboldetector and hence, all the more so, the input of the convolutivedecoder, of the information element carried by Z_(i).

The weighting of the symbols transmitted on the channel is aimed,conventionally, at representing the inter-symbol interference. Theweighting coefficients may be fixed or, preferably, estimated, forexample by means of a learning sequence.

Preferably, said coefficient γ₁ depends on at least one of the items ofinformation belonging to the group comprising:

the signal-to-noise ratio;

the number i of the module considered.

Indeed it can be seen that the greater the value of γ₁, the morereliable is the output of the decoder.

When the symbols are subjected to interleaving at transmission, thedecoder preferably comprises:

de-interleaving means providing for the symmetrical operation of saidinterleaving, inserted between said correction means and said decodingmeans; and

interleaving means providing for an interleaving of said items ofcorrection information that is identical to said interleaving attransmission.

The interleaving and de-interleaving may be done by means of matrices.

The reading and writing increments may be fixed or, more advantageously,may depend on the read or write address.

The invention also relates to a detection and decoding module designedto be implemented in a device for the reception of signals formed by aseries of digital symbols corresponding to the convolutive encoding ofitems of source digital data, said device comprising at least twocascade-connected modules, the module having two inputs, one symbolinput R_(i) and one correction input Z_(i), and three outputs, onesymbol output R_(i+1), one correction output Z_(i+1) and one decodedsymbol output D_(i+1), and comprising:

means for the correction of inter-symbol interference supplied by saidsymbol input R_(i) and taking account of said correction input Z_(i) anddelivering estimated symbols A_(i),1,

means for the decoding of said estimated symbols A_(i),1, performingoperations symmetrical to said convolutive encoding and deliveringdecoded symbols A_(i),2, and

means for the computation of an item of correction information Z_(i+1)from said estimated symbol A_(i),1 and said decoded symbol A_(i),2,

the symbol output R_(i+1) being equal to the symbol input R_(i) delayedby a predefined latency time,

the correction output Z_(i+1) delivering said correction information,and

the decoded symbol output D_(i+1), being supplied with said decodedsymbol A_(i),2 or with a value reconstituted as a function of the codeimplemented.

As indicated here above, a module of this kind in particular may takethe form of an integrated circuit.

The invention further relates to a method for the reception of signalsformed by a series of digital symbols corresponding to the convolutiveencoding of items of source digital data comprising the following steps:

supplying with received symbols R_(i) ;

the correction of inter-symbol interference affecting each of saidreceived symbols, by means of an item of correction information Z_(i),and the delivery of estimated symbols A_(i),1 ;

the decoding of said estimated symbols A_(i),1, entailing operationssymmetrical to said convolutive encoding, and the delivery of decodedsymbols A_(i),2 ;

the computation of said items of correction information Z_(i) from atleast one of said estimated symbols A_(i),1 and at least one of saiddecoded symbols A_(i),2.

The invention can be applied, as indicated here above, to all systems ofdigital transmission implementing a convolutive error correctionencoding operation as shown in FIG. 1.

This system has a digital source 11 that must be transmitted to anaddressee 12.

The term digital source is understood to mean any useful digital signal(sound, image, data) and more generally any set of useful signals, forexample time-division multiplexed and/or frequency-division multiplexedsignals. The addressee 12 may be a single addressee (in the case oftransmission) or a multiple addressee (in the case of broadcasting).Similarly, the transmission line may of course be one-way (as in radiobroadcasting) or two-way (as in radio communications for example).

The source of signal is first of all subjected to a convolutive encoding13 of a type known per se. It may consist, in particular, of a standardsystematic convolutive encoding associating, with each item of sourcedata, at least one determined value by the combination of this item ofdata with at least one of the previous items of source data.

It may also be a code such as one of those described in the patentapplication FR 91 05278. These codes, known as "pseudo-systematic"codes, are characterized by the fact that the source data is transmittedsystematically, in conjunction with at least one encoded item of data orredundancy symbol.

Any other convolutive code may also be used.

The items of data delivered by the encoder 13 are then advantageously(although this is not an obligatory characteristic) subjected to aninterleaving operation 14.

This interleaving operation may be obtained conventionally by means ofan interleaving matrix in which the items of source data are introducedline by line and restored column by column.

The module 14 provides for matrix interleaving. The items of data arewritten, in successive lines, in a memory with a size n_(E) xn_(E), andrestored in successive columns. This technique is described for examplein the article by E. Dunscombe and F. C. Piper, "Optimal interleavingscheme for convolutional coding", in "Electronics Letters", Vol. 25, No.22, October 1989.

As can be seen in this article, the efficiency of the interleaving forsmall values of n_(E) (some tens) is improved if the succession of rowsand columns occurs according to an increment greater than 1, thisincrement being, by necessity, relatively prime with n_(E).

An additional improvement to this technique of interleaving may beimplemented. For it appears to be advantageous that the row-skipincrement, which is always relatively prime with n_(E), should be afunction of the position of the column considered. This makes itpossible to break the rectangular-positioned error packets.

In this case, the interleaving matrix read increments are chosen in sucha way that the minimum distance at output between two symbols that areneighboring at input is as great as possible. These increments may, forexample, be determined empirically.

Other interleaving techniques may of course be used, especially theusual method of write and read increments equal to 1, without departingfrom the framework of the invention. It is also clear that the rows andcolumns may be inverted.

Then, the interleaved signal is transmitted through the transmissionchannel 15. It therefore undergoes a modulation and a filteringoperation 16, according to any appropriate technique, and is thentransmitted through the transmission medium 17.

This transmission medium 17 is generally noise-infested and showsinterference between the symbols transmitted in it.

It must be noted that, according to the invention, the two sources ofdisturbance namely noise and inter-symbol interference (IES) areconsidered in a dissociated manner. In other words, the symbols affectedby IES are in addition noise-infested. The IES is chiefly due todistortion introduced by the filtering elements of the transmissionsystem and the presence of multiple paths induced by the physical mediumof transmission (due to reflections and obstacles, etc.). Thesephenomena lead to disturbances between symbols. The noise induced by thephysical medium gets added to this IES phenomenon and is likely inaddition to modify the items of information borne by the symbols.

FIG. 2 illustrates this approach in the simplified case of interferencerelating to two symbols.

The reference An is given to the symbol transmitted at the instant nT,where 1/T is the speed of modulation (T being the duration of thesymbol). The received signal Sn can be written, after passage in thetransmission channel:

    S.sub.n =Σ.sub.k h.sub.k A.sub.n-K

This signal comprises a term dependent on the symbol A_(n), namely h₀A_(n), but also a term dependent on the symbols transmitted prior to andsubsequently to A_(n). This second term is the inter-symbol interferenceterm. It can be written in the form Σ_(k)≠0,h_(k) A_(n-k). It is due tothe filtering introduced by the channel and may be modelled by a latticewith N=2.sup.υ states, for an IES modelled by a discrete equivalentchannel with a memory υ (two in the case of FIGS. 2 and 3).

In the case of FIG. 2, interference relating to two symbols (A_(n-1) andA_(n+1)) is considered. The effect of the channel may then berepresented by two delay cells 21₁ and 21₂, each with a duration T.

When the symbol An+1 is presented at input of the cell 21₁, the outputof this cell 21₁ is A_(n) and the output of the second cell 21₂ isA_(n-1). The filtering of the channel amounts to the multiplying (22₁ to22₃) of each of these symbols by the corresponding filtering coefficienth_(i). The corresponding terms are then added up (23) to form the signalS_(n).

The architecture of FIG. 2 corresponds to a convolutive encoding withυ=2, the decoding lattice of which is illustrated in FIG. 3,conventionally, this lattice illustrates the possible transitions as afunction of the two preceding values depending on whether the value ofA_(n+1) is 1 (solid lines) or 0 (dashes).

Returning to FIG. 1, the noise-infested signal S_(n) is subjected to afiltering and a demodulation 18, which are symmetrical to the operationsperformed by the module 16.

Then, conventionally, the signal is equalized (19) by the application ofa filtering operation designed to compensate for the effects of thetransmission medium 17. The equalized signal is then de-interleaved(110) symmetrically to the interleaving operation 14.

Finally, the signal is transmitted to a convolutive decoder 111 which,for example, implements an operation of maximum likelihood decoding suchas the Viterbi algorithm to deliver decoded items of data to theaddressee 12.

The essential characteristic of the invention is the iterativeassociation of a symbol detection module with weighted outputs and adecoding module, also with weighted outputs. If we consider the effectof the channel according to the approach presented, the standardequalization (the filtering of the received signal by means of atransversal filter, for example) is replaced by an operation of symboldetection that takes account of the memory effect introduced by thechannel.

Thus, the proposed device includes a detector with weighted input andoutput that estimates the symbols A_(n) from the noise-infested outputof the channel. For example, this detector may be made by using thealgorithm described in the patent document FR 91 05279. It requiresknowledge of the coefficient h_(k) which should therefore be estimatedexternally to the proposed device, for example by means of learningsequences.

FIG. 4 illustrates the general principle of the invention.

Each received sample R_(n), which corresponds to the noise-infestedoutput of the transmission channel, is processed as follows:

a detector 41 of symbols with weighted outputs produces an estimationA_(n),1 of the corresponding transmitted symbol;

the symbols A_(n),1 at output of the detector 41 are de-interleaved (42)to give the symbols A_(k),1 ;

the de-interleaved symbols A_(k),1 are presented to the input of aconvolutive decoder 43 which gives a new estimation A_(k),2 of symbolsA_(k) ;

at output of the decoder 43, there is defined (44) a correction dataelement Z_(k) called an extrinsic data element obtained from theestimations A_(k),1 and A_(k),2 ;

the correction data element Z_(k) is then used, after a re-interleavingoperation 45, in the detector 41.

More specifically, in the embodiment described, the item of data Z_(k)is obtained by difference between A_(k),2 and γ₂ A_(k),1 delivered bythe multiplier 46, where γ₂ is a positive coefficient, Z_(n) is still anestimation of the symbol A_(n), but this time affected by a noise thatis not correlated with the noise that affects A_(n),1 and is slightlycorrelated with the noise affecting R_(n) by means of the interleavingfunction 45.

The extrinsic information Z_(n) is then used by the detector 41 by theaddition or deduction, to or from each arm metric of the latticeassociated with the channel (cf. FIG. 3), of the term γ₁ |Z_(n) |, whereγ₁ is a positive coefficient. The term γ₁ |1|Z_(n) | is deducted (oradded respectively) when the symbol assigned to the arm considered hasthe same sign as Z_(n) (or respectively the opposite sign). The value ofthe coefficient γ₁ depends on both the signal-to-noise ratio and thereliability of the extrinsic information element Z_(n).

The extrinsic information element, assigned the coefficient γ₁ for themultiplier 48, is furthermore deducted (47) from the output of thedetector 41 before being presented to the input of the decoder 43.Indeed, since the extrinsic information element Z_(n) has been producedby the decoder 43, it cannot be reused by this decoder 43.

The symbol detector 41 takes its decisions according to a Viterbi typealgorithm. All the operations performed by this detector correspond tothose described in the patent FR 91 05279. The difference, as comparedwith a convolutive decoding algorithm, lies in the computation of thetransition metrics or branch metrics, which is done differently.

Each node of the lattice is represented by a possible state of thechannel at a given instant nT. For an IES value relating to K1 symbolssubsequent to the symbol considered and K2 symbols prior to the symbolsconsidered, namely for:

    S.sub.n =Σ.sub.k=K1 to K2 h.sub.k A.sub.n-k

the state of the channel at t=nT is given by the(K1+K2)-uplet(A_(n+K1-1) . . . A_(n+1) A_(n) A_(n-1) . . . A_(n-K2)).The knowledge of the state of the channel at t=nT as well as the valueof A_(n+K1) (symbol entering the channel) then provides knowledge of itsstate at t=(n+1)T.

Thus, for each node j of the lattice taken at the instant t=nT, twotransition metrics L_(j) 0 and L_(j) 1 are computed. They correspond tothe two possible transitions (A_(n+K1) =0 and A_(n+K1) =1). They havethe form:

    L.sub.j 0=(R.sub.n -R.sub.j 0)2 and L.sub.j 1=(R.sub.n -R.sub.j.sub. 1)2

where R_(n) represents the sample actually received at input of thedetector and R_(j) 0 (or R_(j) 1 respectively) is the theoretical valueof the sample that is not noise-infested when the state of the channelcorresponds to the node j of the lattice and when A_(n+K1) =0 (orA_(n+K1) =1 respectively). R_(j) 0 (or R_(j) 1 respectively) thereforehas the form: Σ_(k=K1) to K2 h_(k) A_(n-k). If A_(n+K1) =0, then thevalue is R_(j) 0 and if A_(n+K1) =1 then the value is R_(j) 1.

In practice, the following simplified expression is used to compute themetrics:

    L.sub.j 0=|R.sub.n -R.sub.j 0| and L.sub.j 1=|R.sub.n-R.sub.j 1|

The performance characteristics of the detection are then very close tothose obtained with the previous expression and the implantation of thecomputation on silicon is greatly simplified.

Apart from this computation of the metrics, the computation algorithm issimilar in principle as well as in its implementation to a convolutivedecoding algorithm.

In practice, the receiver shown in FIG. 4 induces a computation latency(processing time of the detector 41, the decoder 43, the interleavingoperation 45 and the de-interleaving operation 42). It is therefore notpossible to obtain instantaneous knowledge of Z_(n) when R_(n) isreceived in the detector 41. Consequently, according to an essentialcharacteristic of the invention, the process implemented is iterative.

This process is illustrated schematically by FIG. 5. For each symbolreceived Rn (reception step 51), at least two iterations 52 areperformed.

Each iteration 52 includes a step 521 for the correction of inter-symbolinterference. As indicated here above, this correction implements aconvolutive decoding technique and takes account of an item of extrinsiccorrection information Z_(n) and items of data h_(i) representing thetransmission channel.

The items of information Z_(n) are computed during the step 524.Naturally, at the first iteration, no particular value of Z_(n) isknown. Consequently, this value is set at a neutral value, namely at avalue having no effect on the computations.

After the correction 521, the estimated symbols are de-interleavedduring the de-interleaving step 522 (if an interleaving operation hasbeen implemented at transmission) and then decoded according to astandard technique of convolutive decoding 523 and for example accordingto the technique described in the French patent application FR 91 02579.

Then, the item of extrinsic information Z_(k) is computed (524) from theitem of data 525 detected by the correction step 521 and the item ofdata 526 decoded by the decoding step 523. Then, the items ofinformation Z_(k) are re-interleaved (527) symmetrically with thede-interleaving 522 to give the item of information Z_(n). The item ofinformation Z_(n) is taken into account in the correction step 521 ofthe next iteration.

At the last iteration, the step 524 for the computation of Z_(k) is notimplemented. The decoded symbol D_(k) is delivered by the decoding step523.

The corresponding reception device may advantageously be made in amodular fashion, by associating one module with each iteration. FIG. 6illustrates a device of this kind comprising four modules.

The first module 61 of the system receives (R)_(n), the symbol receivedby the device and delivers firstly (R)_(n),2, which is equal to (R)_(n)assigned a delay corresponding to the latency of the module and,secondly, (Z)_(n),2 the extrinsic information computed by the module 61.Each of the following modules 62 to 64 receives the delayed symbol(R)_(n),i and the item of extrinsic information (Z)_(n),i. The lastmodule 64 delivers the decoded symbol (D)_(n).

It will easily be understood that the performance characteristics of thedevice in terms of error rate are a function of the number ofcascade-connected modules.

Advantageously, all the modules are identical. FIG. 7 illustrates anexemplary embodiment of a module of this kind. More specifically, it isthe module corresponding to the p^(th) iteration. It includes:

two inputs: (R)_(p) and (Z)_(p),

three outputs: (R)_(p+1), (Z)_(p+1) and (D)_(p+1).

The input (R)_(p) is equal to the output of the channel delayed by thelatency of p-1 modules.

The input (Z)_(p) comes from the previous module and is set, for p=1, ata neutral value, namely a value that has no influence on the output ofthe detector.

The output (R)_(p+1), equal to the input (R)_(p) delayed by the latencyof a module, is unused if the iteration is the last one of the process(p=P, P being the total number of modules).

The output (Z)_(p+1) is the extrinsic information element prepared atthe p^(th) iteration. This output is also unused if p=P.

The output (D)_(p+1) which is the decoded item of data is unused by theP-1 first modules, and only the items of data coming from (D)_(p+1) aregiven to the addressee.

The elements of this module that have already been described withreference to FIG. 4 are not described again, and bear the samereferences. It will be noted that, as compared with the schematicdiagram of FIG. 4, different delays are added to take account of thelatencies of the different elements.

Thus, the extrinsic information (Z)_(p) used for the difference 47 mustbe delayed (71) by a duration equal to the latency L₁ of the detector41. This is also the case with (A)_(p),1 which must be delayed by thelatency L₂ of the decoder 43. Finally, (R)_(p) must be delayed (73) bythe total latency introduced by a module L_(T) in order that (R)_(p+1)and (Z)_(p+1) may be presented at the same instant to the input of thefollowing module. This latency L_(T) is equal to the sum of the latencyof the detector 41, L₁, the latency of the decoder L₂ and the latencyintroduced by the interleaving operations 45 and de-interleavingoperations 42 L_(ED).

The performance characteristics of the device of the invention have beensimulated and are illustrated in FIG. 8. The item of source data isencoded by a recursive systematic convolutive code (CSR code) with aconstraint length K=5 and efficiency R=1/2. The encoded items of dataare sent on the transmission channel after a non-uniform interleaving ina matrix of 64 rows and 64 columns. The modulation used is a binaryphase-shift keying (BPSK) modulation.

The multiplier coefficients h_(i) are real and constant and known to thesymbol detector but the principle is applicable when they are complexand variable. For the results presented hereinafter, the coefficientsh_(i) have been fixed at:

h₀ =√0.45, h₁ =√0.25, h₂ =√0.15, h₃ =√0.10, h₄ =√0.05

They verify Σ_(i=0) to 4 h_(i) 2=1. The transmission channel is alsodisturbed by a Gaussian white additional noise.

At output of the channel, the received samples Rn are quantified on 8bits. The symbol detector and the channel decoder use the algorithmpresented in the patent FR 91 05279. The working of the decoder is alsorepresented by a 16-state lattice. The values of coefficients γ₁ and γ₂have been determined empirically.

The results shown use four modules. The curves C₁ and C₂ represent theerror rate at output of the transmission system, respectively withoutand with channel encoding, when there is no inter-symbol interference:C₃ gives the error rate in the classic case where the symbol detectorgives a binary decision (firm decision) and the curves C_(4i) show theresults obtained with the proposed device after the i-order iteration.

These curves therefore show that, at an error rate of 10⁻⁵, the use of asymbol detector with weighted output makes it possible to gain 1.6 dB ascompared with a firm decision detector and the three additionaliterations provide an additional gain of about 3 dB. When the number ofiterations increases, the performance characteristics approach thoseobtained on an encoded Gaussian channel.

What is claimed is:
 1. Device for the reception of signals formed by aseries of digital symbols corresponding to the convolutive encoding ofitems of source digital data characterized in that it comprises:pcascade-connected detection and decoding modules M₁ to M_(p), p beinggreater than or equal to 2, each of said modules M_(i)including:inter-symbol interference correction means supplied by asymbol input R_(i) and delivering estimated symbols A_(i),1 withweighted values; means for the decoding of said estimated symbolsA_(i),1 performing operations symmetrical to said convolutive encodingand delivering decoded symbols A_(i),2 with weighted values; and meansfor the computation of an item of correction information Z_(i+1) fromsaid estimated symbol A_(i),1 and said decoded symbol A_(i),2, saidmeans for the correction of each of said modules M_(i), except for thefirst module M₁, taking account of at least one item of correctioninformation Z_(i) determined by the previous module M_(i-1).
 2. Deviceaccording to claim 1, characterized in that:each of said modules M_(i)has two inputs, one symbol input R_(i) and one correction input on whichcorrection information Z_(i) may be received, and three outputs, onesymbol output R_(i+1), one correction output and one decoded symboloutput D_(i+1) ; the symbol input R_(i) of the module M_(i) beingconnected to the symbol output R_(i) of the module M_(i-1) or i greaterthan 1 and being supplied with the symbols received for i equal to 1;the correction input of the module M_(i) being connected to thecorrection output of the module M_(i-1) for i greater than 1 and beingsupplied with a neutral value that does not affect the correction for iequal to 1; the symbol output R_(i+1) of the module M_(i) being equal tothe symbol input R_(i) of said module M_(i) delayed by the latency timedictated by the module M_(i) ; the correction output delivering saidcorrection information; and the decoded symbol output D_(i+1) beingunused for the modules M₁ to M_(p-1) and being supplied with the decodedsymbol A_(p),2 of the module M_(p) if the code implemented is asystematic code and, if not, with a corresponding reconstituted value.3. Device according to any of the claims 1 and 2, characterized in thatsaid means (41) for the correction of the inter-symbol interference aresupplied with an item of correction information Z_(i) representing adifference (44) between each decoded symbol A_(i),2 and thecorresponding estimated symbol A_(i),1.
 4. Device according to claim 3,characterized in that said means to compute the item of correctioninformation comprise means (46) for the multiplication of saidestimation symbols A_(i),1 by a positive weighting coefficient γ₂delivering weighted symbols, and subtraction means (44) computing thedifference between each decoded symbol A_(i),2 and the correspondingweighted symbol and delivering said item of correction informationZ_(i).
 5. Device according to any of the claims 1 to 4, characterized inthat said means (41, 47, 48) for the correction of inter-symbolinterference perform the following operations:the computation (41) of acorrection value V_(i) =γ₁ |Z_(i) |, where γ₁ is a positive coefficientand where | | represents the absolute value operator; for each arm ofthe lattice corresponding to said convolutive code, the determining (41)of a detected symbol A_(i),0, comprising the following steps:if thesymbol assigned to said branch has the same sign as said item ofcorrection information Z_(i), the subtraction, from the metricassociated with said arm, of said correction value V_(i) ; if the symbolassigned to said arm has a sign opposite that of said item of correctioninformation Z_(i), the addition, to the metric associated with said arm,of said correction value V_(i) ; the subtraction (47), from saiddetected symbol A_(i),0, of the value γ_(i) Z_(i).
 6. Device accordingto claim 5, characterized in that said coefficient γ₁ depends on atleast one of the items of information belonging to the groupcomprising:the signal-to-noise ratio; the number i of the moduleconsidered.
 7. Device according to any of the claims 1 to 6, applied tothe reception of symbols subjected to an interleaving operation attransmission, characterized in that it comprises:de-interleaving means(42) providing for the symmetrical operation of said interleaving,inserted between said correction means and said decoding means; andinterleaving means (45) providing for an interleaving of said items ofcorrection information that is identical to said interleaving attransmission.
 8. Detection and decoding module designed to beimplemented in a device for the reception of signals formed by a seriesof digital symbols corresponding to the convolutive encoding of items ofsource digital data, said device comprising at least twocascade-connected modules, characterized in that it has two inputs, onesymbol input R_(i) and one correction input Z_(i), and three outputs,one symbol output R_(i+1), one correction output Z_(i+1) and one decodedsymbol output D_(i+1), and in that it comprises:means (41, 47, 48) forthe correction of inter-symbol interference, supplied by said symbolinput R_(i) and taking account of said correction input Z_(i) anddelivering estimated symbols A_(i),1, means (43) for the decoding ofsaid estimated symbols A_(i),1, performing operations symmetrical tosaid convolutive encoding and delivering decoded symbols A_(i),2, andmeans (44, 46) for the computation of an item of correction informationZ_(i+1) from said estimated symbol A_(i),1 and said decoded symbolA_(i),2, the symbol output R_(i+1) being equal to the symbol input R_(i)delayed by a predefined latency time, the correction output Z_(i+1)delivering said correction information, and the decoded symbol outputD_(i+1) being supplied with said decoded symbol A_(i),2 or with a valuereconstituted as a function of the code implemented.
 9. Method for thereception of signals formed by a series of digital symbols correspondingto the convolutive encoding of items of source digital data comprisingthe following steps:supplying with received symbols R_(i) ; andperforming at least two iterations of the following steps:correctinginter-symbol interference affecting each of said received symbols, bymeans of an item of correction information Z_(i), and the delivery ofestimated symbols A_(i),1 ; decoding said estimated symbols A_(i),1entailing operations symmetrical to said convolutive encoding, and thedelivery of decoded symbols A_(i),2 ; and computing said items ofcorrection information Z_(i) from at least one of said estimated symbolsA_(i),1 and at least one of said decoded symbols A_(i),2.